Reenable git wire protocol v2 behind feature flag

Part of gitlab#27828 (closed)

This change replaces the gatekeeping of git wire protocol v2 from a non-configurable configuration field (😶) with a feature flag.

The gitaly-ssh command was also modified to provide the feature flag when the request specifies protocol v2. Otherwise, some of the SSH service unit tests would not be able to properly enable the feature flag.